Thyroid Gland

The thyroid gland sets the body's metabolic pace, producing hormones that influence how nearly every tissue uses energy. It also plays a supporting role in calcium balance.

Structure and Location

The thyroid is a butterfly-shaped gland in the front of the lower neck, just below the larynx (voice box) and wrapping around the front and sides of the trachea. It has two lobes, right and left, joined across the midline by a bridge of tissue called the isthmus, which typically crosses the second and third tracheal rings. The gland is highly vascular and richly supplied with blood.

Microscopically, the thyroid is built from millions of spherical follicles, the structural and functional units of the gland. Each follicle is a ring of cuboidal follicular cells (thyrocytes) surrounding a core of stored protein-rich colloid. Scattered between follicles are parafollicular cells (C cells).

Key Functions

Follicular cells take up iodine and use it to synthesize the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones regulate basal metabolic rate, heat production, heart rate, and normal growth and brain development, especially in infancy. Their production is governed by thyroid-stimulating hormone (TSH) from the pituitary, which in turn responds to TRH from the hypothalamus.

The parafollicular C cells secrete calcitonin, a hormone that lowers blood calcium by inhibiting bone breakdown, although in humans its role is relatively minor compared with parathyroid hormone.

Clinical Relevance

Thyroid disorders are common. Hypothyroidism (underactivity, often from Hashimoto's thyroiditis) causes fatigue, weight gain, and cold intolerance, while hyperthyroidism (overactivity, often from Graves' disease) causes weight loss, rapid heartbeat, and anxiety. Iodine deficiency can cause goiter (gland enlargement). Thyroid nodules are frequent and usually benign, but some are cancerous. Because the gland sits near the recurrent laryngeal nerves and parathyroid glands, thyroid surgery requires careful technique.

*This content is educational and not a substitute for professional medical advice.*